Is there an in-universe explanation for the purring sound of the Enterprise warp core?

Question

The purring sound of the Enterprise warp core is iconic (for example https://news.avclub.com/read-this-behind-the-scenes-of-sci-fi-s-most-soothing-1798247701). Is there an in-universe explanation for the components or processes specifically responsible for this sound?

I'm asking specifically about the throbbing of the warp core, not the engines.

This YouTube video lets you hear the throbbing sound well:

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It's interesting that some "ambient" videos on YouTube don't present the throbbing sound as distinctly. Which makes me wonder why the film makers sometimes included it and sometimes didn't. Either it's accidental, or the throb is present sometimes but not others for a reason. I'm wondering what that reason might be.

It's possible the throbbing sound has nothing to do with the warp core and is due to some other part of the engine room. But in scenes where the rings of light surrounding the core flash in a way to emulate motion, the flashing seems to follow the same period as the throbbing sound.

This video shows the synchrony between the flashing tubes and the throbbing sound (although the throb sound is very subtle):

Answer 1

According to the EU novel Captain's Glory (written by William Shatner), the sound you're hearing is the sounds of matter-antimatter collision, contained within a forcefield.

At once he heard the thrum of the matter-antimatter reactor power up as the vessel’s shields went from standby navigational mode to battle strength.

The EU novel The Shocks of Adversity suggest that mixed in with that is the sound of the warp plasma relays relaying warp plasma.

With the press of a single button, the low steady thrum of the giant matter/antimatter reactor was joined by two higher alternating notes, creating a steady rhythm that, judging from the smile Scotty wore as he turned from the situation console, was music to his ears. “Isn’t that the most beautiful sound ye ever heard, sir?”

Kirk shared the engineer’s smile. “Definitely among the top three,” he agreed. The warp plasma relays had been restored and realigned, and the mellifluous sounds emitting from the engines confirmed this. “Anything else left outstanding, Scotty?”

Answer 2

The pulsating thrum of the engine could be a beat, that is, the sound resulting from the alternation of constructive and destructive interference of sound waves. But what generates the sound waves to begin with, I do not know. You can hear a beat demonstrated on this brief YouTube film.